Method for single crystal growth



Sept; 1, 1959 D. A. JENNY ETAL 2,902,350

METHOD FOR SINGLE CRYSTAL GROWTH Filed Dec. 21, 1954 I 8 K yA A I IN VENTORS .o/sm/n/ 4. JEN/V) aw HrTaRA/EY I United States Patent NIETHOD FORSINGLE CRYSTAL GROWTH Dietrich A. Jenny, Pennington, and Robert V.Jensen, Trenton, N.J., assignors to Radio Corporation of America, acorporation of Delaware Application December 21, 1954, Serial No.476,734 4 Claims. (Cl. 23-301) This invention relates to improvedmethods for growing single crystals by zone-melting and moreparticularly to growing such crystals without the necessity of aninitial seed crystal.

Heretofore, in the preparation of semi-conductor materials, for example,it has been customary to grow a single crystal of a substance by meltingthe substance and contacting a seed crystal of the substance to the meltand then either slowly withdrawing the seed or slowly moving the meltingzone of heat along the material and away from the seed. In either casethe molten substance freezes-out on the seed crystal and grows as asingle crystal therefrom along an axis determined by the orientation ofthe seed crystal. In the moving zonemelting system just described, itis, of course, possible to keep the heating zone stationary while movingthe material to be melted and re-crystallized.

There are many reasons why it is desirable to grow single crystalswithout requiring a seed crystal. The seed crystal is a valuable crystalwhich could be used, in the case of germanium and silicon, to make alarge number of semi-conductor devices. This is especially true sincethe seed crystal must be large enough to be securely gripped by a chuckor other supporting apparatus. The use of a seed crystal to growadditional crystals therefore represents waste.

Difficulty has been encountered in the past in growing single crystalsbecause of the formation of small nucleating centers in the growingcrystal resulting in the growth of disoriented crystals. This is truewhether the crystal is being grown from a seed crystal or not. Sourcesof such undesired nucleating centers are discontinuities in thecontainer Walls. Such crystals tend to grow in a direction normal to thewalls of the container. If the advancing interface between the liquidand solid phases of the material is at right angles also to the walls ofthe container and hence parallel to or coincident with the direction ofgrowth of crystals from these nucleating centers, the extent of theirgrowth into the principal crystal is considerable.

It is therefore an object of this invention to obviate the necessity fora seed crystal in growing a single crystal of a substance by thezone-melting technique.

Another object is to reduce the formation and development of multiplenucleating centers in 'the process of growing single crystals.

A further object of the invention is to control the shape of theinterface between the solid and liquid phases of a growing singlecrystal so as to minimize the effect of accidental nucleation ofsecondary crystals with respect to the growing of a single crystal.

Another object of the invention is to estbalish an interface inclinedwith respect to the vertical between the solid and liquid phases of agrowing single crystal in a zone-melting furnace while maintaining thehorizontal shape of the interface convex.

An additional object is to grow without seed a single crystal of asubstance in a zone-melting furnace while 1 2,902,350 Patented Sept. 1,1959 maintaining the shape of the interface between the liquid-solidphases of the growing single crystal as horizontally convex andvertically inclined.

Another object is to provide improved methods of growing ingots ofsingle crystal material.

These and other objects and advantages of the invention areaccomplished, for example, by so heating an ingot of the substance to bemelted and re-grown as a single crystal as to establish a verticaltemperature gradient from the top to the bottom of the substance. Thisestablishes, in turn, an inclined interface between the liquid-solidphases of the ingot with the length of the molten phase being greater atthe bottom than at the top of the ingot. By exposing the sides of theingot to a heat source for a longer period than the bottom thereof, thesides remain molten longer. Thus the center of the melt freezes-outfirst and the sides last, to establish a horizontally convex shape ofthe solidifying interface. Thus by establishing the liquid-solidinterface at an angle with respect to both the bottom and side walls ofthe container, crystals originating from undesired nucleating centersthereat meet and stop growing at the advancing boundary of the principalcrystal while still relatively small. Finally, by reducing thetemperature of a small area of the melt to about the freezing pointthereof desired small area nucleation takes place thereat and singlecrystal growth is initiated therefrom.

The invention will be described in greater detail with reference to thedrawings in which:

Figure 1 is a partially schematic cross-sectional side view of ahorizontal zone-melting furnace used accord ing to the method of thisinvention,

Figure 2 is a partial cross-sectional plan view of the furnace of Figure1 taken along the line 2-2 thereof,

Figure 3 is a perspective view of another embodiment used in the methodof applicants invention, and

Figure 4 is a partial view in section of the embodiment shown in Figure3.

Like numerals represent similar elements throughout the drawings.

A single crystal of a substance such as germanium or silicon for examplemay be grown without a seed crystal utilizing the apparatus shown in thedrawings. Aningot 2 of germanium is placed within an elongated carbonboat 4. The side walls of the boat'4 should be low enough to expose asmuch of the ingot 2 as possible while serving to support the ingot inits molten phase. Inasmuch as only a small portion of the ingot 2 ismolten at a given time surface tension will permit a relatively largevertical exposure of the molten phase without side-wall support. As willbe explained later this is an important feature of the invention.

The elongated boat or crucible 4 is placed within a refractory tube 6that may be of quartz and is provided with inert gas inlet and outletmeans 8 and 10. The tube 6 and the crucible assembly are surrounded by azonemelting furnace 16 which extends longitudinally along a relativelysmall portion of the length of the assembly.

The zone-melting furnace 16 may be heated by any convenient means suchas the electric resistance heating elements 14 connected to anyconvenient power source, not shown. The furnace 16 is adapted to travelalong the length of the boat 4 from one end to the other at a controlledrate. Any convenient means may be pro vided to propel the furnace 16along the tube 6. For example, the furnace 16 may be supported by thetwo brackets 18 and 20. The bracket 18 is suspended from the rail 22while the bracket 20 rests on the screw 24. The screw 24 may be rotatedby the motor 26 to propel the furnace in a desired direction at acontrolled speed. Alternatively, the furnace 16 may be held stationaryand the. crucible 4 may be adapted to progress through the furnace.

In typical apparatus the elongated vessel 4 may be about 24" long. and1" in width, for example, and the germanium ingot 2 may weight about 1kilogram. In operation, a protective gas such as hydrogen or an inertgas is maintained within the tube 6.

In operation the motor 26 is started and the furnace 16 is driven slowlyalong the length of the tube 6 and the crucible 4- The electricalresistance heating elements are supplied with power so as to raise thetemperature within the furnace to at least 50 C. above the melting pointof the material of the ingot 2, which in the case of germanium is about900 C. Thus as the crucible 4 and the ingot 2 enter the furnace theingot 2 becomes molten- The temperature of the molten zone is notcritical except that it should be substantially higher than the meltingpoint of the ingot 2 so as to insure complete melting of the entire massof the ingot within the furnace 16. Furthermore, if the temperature ofthe molten zone is kept at a point only slightly above the melting pointof the ingot 2, relatively small solid crystallites of the material mayremain in the molten zone without melting. These crystallites mayprovide additional nucleating centers as the zone progresses andinterfere with the growth of a large single crystal.

As the furnace 16 is driven along the length of the crucible 4, acontinually changing portion of the ingot 2 is melted and re-frozen. Dueto the fact that the heat source (electric resistance elements 14) islocated beneath the crucible 4 and the ingot 2 the bottom portions ofthe ingot 2 melt first and freeze-out last. The top portions of theingot 2 melt last and freeze-out first. This means therefore that theinterface between the molten-solid phases of the ingot 2 is at aninclination with respect to the perpendicular to the floor of thecrucible 4. As pointed out previously, small nucleating centersoriginating at discontinuities of the crucible wall will tend to growcrystals in a direction normal to the crucible walls. By having theinterface between the molten-solid phases reach such nucleating centersas soon as possible this undesired crystal growth can be checked whilethe crystals are relatively small. This is accomplished by the inclinedinterface between the moltensolid phases of the ingot 2 as provided bythe invention as far as the floor of the crucible 4 is concerned.

One way to instigate nucleation of a crystal in a molten mass is to coola small area of the mass to the point of solidification and then tocontinue freezing-out successive portions of the molten mass adjacent tothe small nucleating area. The invention takes advantage of thistechnique and also of the inclined interface between the molten-solidphases of the ingot 2 to grow a large single crystal Without thenecessity of a seed. To do this a stream of inert gas is directedagainst the forward molten surface of the ingot 2 so as to strike theingot surface at the point A. The flowing inert gas reduces thetemperature of the melt at point A to about the freezing point of theingot 2 whereupon a small nucleating center is formed. As the furnace 16continues to pass over the tube 6 and the crucible 4, the slantedinterface 28 moves along likewise resulting in the freezing-out of thetop portions of the melt first. Thus the nucleating center is formed bythe cooling gas stream at point A and then the portions of the meltimmediately adjacent thereto are successively frozen-out by the movementof the melting zone to grow a single large crystal from the nucleatingcenter.

The ingot 2 is also in contact with the side Walls of the crucible 4.Therefore any discontinuities in these walls will also tend to formnucleating centers and hence undesirable crystal growth. To inhibit suchcrystal growth the invention causes the interface between theliquid-solid phases to be horizontally convex. Referring to Figure 2 theinterface 28 is shown as being convex with the. outermost portions ofthe ingot 2 freezing out after the center of the ingot has solidified.Since crystal growth tends to progress in a direction normal to theinterface, a convex shaped interface, therefore, provides a freezing-outfront such that undesired non-uniform crystal growth tends to extendtoward the outer edges of the ingot and not to continue along the lengthof the ingot. The convex shape of the ingot 2' is achieved according tothe invention by causing the sides of the ingot to receive more heatthan the center portions. As shown in Figure 2, the side portions of theingot 2 which are exposed above the crucible walls, are exposed todirect radiation fromrelatively long lengthsof the heating elements 14during the passage of the ingot 2 through the furnace 16. This meansalso that these side portions of the charge adjacent the entrance andexit of the heating chamber of the furnace are raised to a highertemperature than other portions of the charge 2 which are relativelyprotected from the heat by crucible 4. Hence the side portions of theingot 2 are maintained molten longer than the center portions thereofresulting in the desired convex. interface between the liquid-solidphases of the ingot.

The rate of. travel of the furnace to most advantageouslyachieve'maximum crystal growth is only slightly variable. Severalfactors must be considered such as the rate of maximum crystal growth,the kind of material involved, and the temperature of the melt. Ingrowing a, single; crystal of germanium, for example, a speed of about 1to 3' mm. per minute gives satisfactory results when the molten zone ismaintained at about 50 C. above the melting point of germanium.

When the furnace 16 has traversed the entire length of the crucible 4,the entire ingot 2 has been converted to a single crystal and theprocess is complete.

The method of heating the zone furnace 16 is not critical. Electricresistance elements such as bars of silicon carbide known: commerciallyas Globars are suitable.

In zone-melting metals such as germanium and silicon it is. desirableto'provide a protective atmosphere such as hydrogen or an inert gas toprevent oxidation of the heated metal and to minimize the introductionof impurities into the metal. The presence of a special at,- mosphere:is. desirable in many zone-melting applications; however, the provisionof a protective atmosphere is not an essential part of the invention andmay be omitted in certain instances such as when zone-melting chemicallystable salts or oxides. In such cases a stream of air may be directedagainst the forward surface of the molten ingot 2 in order toestablishthe nucleus for single crystal growth.

Figures 3 and 4 illustrating an alternative embodiment of the inventionwherein the inclining interface 28 between the molten-solid phases ofthe ingot 2 may be achieved. In this embodiment the wall 30- of thefurnace has an aperture which may serve either for the entrance or exitof the tube 6 and the vessel 4 containing the ingot 2. The wall 32 whichis opposite the wall 30 likewise contains an aperture for the samepurposes. The wall 30 is slanted toward the opposite wall 32 so as toprovide a vertically tapering furnace chamber. The uppermostportion ofthe furnace 16 is thus smaller in length between the walls 30 and 32which contain the exit and entrance apertures. If desired, of course,both walls 30 and 32 could be. sloped toward each other to obtain thesame internal furnace chamber shape. Two electric resistance heatingelements 34 are provided in the bottom of the furnace 16 while oneelement 36. is located in the narrowed top portion. This permits notonly more rapid melting of the ingot but also provides a better controlof the vertical temperature gradient inside the furnace 16. Thearrangement of the heating elements also provides additional undersideheating of' the ingot 2. As

the ingot 2 moves through the furnace 16 the top portion IC i less heatthan the bottom portion. This is because of the arrangement of theheating elements and because of the shortness of the travel path in theupper part of the furnace in comparison with the longer length of travelat the bottom part of the furnace. This means that the top portions ofthe ingot 2 melt last and freezeout first in comparison with the bottomportions. This in turn provides the desired slanted interface betweenthe liquid-solid phases of the ingot. It is evident, of course, thatnucleation could be initiated at the interface by directing a stream ofinert gas thereagainst. Likewise the horizontal shape of the interfacecan be rendered convex by the same technique as described in connectionwith the furnace of Figure 1. It will be noticed that the furnace shownin Figure 3 provides an extremely sharp gradient. Such a gradient is notonly essential for single crystal growth but for homogeneity as well.

The practice of the invention is not limited to the particular materialsdescribed heretofore. It is equally applicable to zone-melting of othermaterials, such as metals generally and salts. Neither is the practiceof the invention limited to any particular type of zone furnace. Anyknown means of providing a relatively narrow appropriately shaped movingzone of relatively high heat may be utilized.

What is claimed is:

1. In a method of growing a single crystal by horizontally zone-meltinga horizontally disposed elongated body of material, the steps of:melting an end portion of said elongated body by applying more heat tothe bottom than to the top thereof to establish a vertical temperaturegradient therein whereby the uppermost parts of said portion of saidelongated body become molten last and solidify first upon temperaturedecrease, the interface between the molten and solidified portions beingthereby inclined, reducing the temperature of a predetermined area of anupper portion of said molten portion to about the freezing point thereofto establish an area of nucleation, and melting and freezing-outsuccessive portions of said elongated body commencing with thoseportions adjacent to said predetermined area whereby a single crystal ofsaid material is grown from said area of nucleation.

2. The invention according to claim 1 wherein said area of said moltenportion is reduced to the freezing point thereof by directing a streamof inert gas thereagainst.

3. The invention according to claim 2 whereby successive molten portionsof said elongated body are frozen-out by providing horizontal relativemotion between said molten portions and the source of said heat.

4. In a method of growing a single crystal by horizontally Zone-meltinga horizontally disposed elongated body of material, the steps of:melting a portion of said elongated body, establishing a verticaltemperature gradient in said elongated body decreasing toward the topthereof such that the top of said molten portion is at a lowertemperature than the bottom thereof, establishing a horizontaltemperature gradient in said elongated body in creasing outwardlyradially from the center thereof such that the side portions of saidmolten portion are at a higher temperature than the center portionthereof, whereby the interface between said molten portion and theadjacent solid portions of said elongated body is vertically inclinedand horizontally convex, reducing the temperature of a smallpredetermined area of an upper portion of said molten portion to aboutthe freezing point thereof for initiating single crystal growth at saidinterface, and melting and freezing-out successive portions of saidelongated body whereby a single crystal of said material is grown.

References Cited in the file of this patent UNITED STATES PATENTS1,450,464 Thomson Apr. 3, 1923 1,738,307 McKeehan Dec. 3, 1929 2,615,060Marinace et a1 Oct. 21, 1952 2,679,080 Olsen May 25, 1954 2,789,039Jensen Apr. 16, 1957 OTHER REFERENCES Schumacher: Ultra Pure MetalsProduced by Zone- Melting Technique, in Journal of Metals, November1953, pp. 1428-29.

Pfann: Journal of Metals, vol. 4, July 1952, page 747-753.

1. IN A METHOD OF GROWING A SINGLE CRYSTAL BY HORIZONTALLY ZONE-MELTINGA HORIZONTALLY DISPOSED ELONGATED BODY OF MATERIAL, THE STEPS OF:MELTING AN END PORTION OF SAID ELONGATED BODY BY APPLYING MORE HEAT TOTHE BOTTOM THAN TO THE TOP THEREOF TO ESTABLISH A VERTICAL TEMPERATUREGRADIENT THEREIN WHEREBY THE UPPERMOST PARTS OF SAID PORTION OF SIADELONGATED BODY BECOME MOLTEN LAST AND SOLIDIFY FIRST UPON TEMPERATUREDECREASE, THE INTERFACE BETWEEN THE MOLTEN AND SOLIDIFIED PORTIONS BEINGTHEREBY INCLINED, REDUCING THE TEMPERATURE OF A PREDETERMINED AREA OF ANUPPER PORTION OF SAID MOLTEN PORTION TO ABOUT THE FREEZING POINT THEREOFTO ESTABLISH AN AREA OF NUCLEATION, AND MELTING AND FREEZING-OUTSUCCESSIVE PORTIONS OF SID ELONGATED BODY COMMENCING WITH HTOSE PORTIONSADJACENT TO SAID PREDETERMINED AREA WHEREBY A SINGLE CRYSTAL OF SAIDMATERIAL IS GROWN FROM SAID AREA OF NUCLEATION.